White Grubs (May, June and Japanese Beetles)

Larvae of white grubs feed on roots of conifer seedlings and transplants. Damage includes seedling discoloration (reddish brown) in late summer to early fall.
White Grubs (May, June and Japanese Beetles) - Articles

Updated: November 21, 2017

White Grubs (May, June and Japanese Beetles)

White grub damage to seedlings. Courtesy of Jim Stimmel, PDA

Phyllophaga sp., Polyphylla sp., Popillia japonica Newman

Hosts

  • All conifers

Damage Potential

  • Moderate–severe on seedlings. Note: Balled-and-burlapped trees are regulated for the Midwest and western United States and parts of Canada, according to the U.S. Japanese Beetle Harmonization plan.

Symptoms and Signs

  • Seedlings and young conifer saplings discolored (reddish brown) in late summer to early fall, leading to eventual death
  • Lateral and taproots chewed off or girdled
  • Small holes, ¼–1⁄3 inch (6–8 mm) in diameter, in soil surface

Causes of Similar Symptoms

  • Drought stress
  • Phytophthora root rot
  • Pales or eastern pine weevil feeding

Identification

Larvae of several species of scarab beetles, including the Japanese beetle, feed on roots of conifer seedlings and transplants. Collectively, the larvae are referred to as white grubs and are similar in appearance. Mature larvae are 1–2 inches (25–55 mm) in length. These C-shaped, grayish-white grubs have tan heads and visible jointed legs. The posterior end of the grub is enlarged and frequently darkened. Most species have sparse yellow hairs on the body. All white grubs have a set of minute spines around the anus. The location of these spines, known as the raster pattern, is used to identify some species.

Adults are dark brown to black beetles up to an inch (25 mm) in length. The common Japanese beetle adult is more colorful and familiar to anyone who gardens. Adult Japanese beetles have rarely been reported to feed on tender bark during a serious outbreak (Figure 1).

Figure 1. Adult Japanese beetle feeding on conifers (rare). Courtesy of Sandy Gardosik, PDA

Calendar of Activities

Biology and Life Cycle

Scarab larvae, or white grubs, overwinter in the soil (Figure 2). In spring, they move to the upper 4 inches of soil and resume feeding on the roots of almost every plant they encounter. When fully grown, these grubs create soil-encrusted cells before pupating near the soil surface. Adults begin to emerge in May and June. After feeding and mating, females deposit eggs just below the soil surface. As larvae emerge from the eggs, they immediately begin to burrow underground. Adequate soil moisture is required for survival, and drought periods will significantly decrease the population due to desiccation of young larvae.

Figure 2. June beetle larva. Courtesy of Steven Katovich, USDA Forest Service, Bugwood.org (#2121096)

Through the summer, the larvae feed in the top 4 inches of soil. They may travel considerable distances in search of plant roots on which to feed. Their strong mouth-parts are capable of cutting off taproots of smaller seedlings. When grubs feed on conifer seedlings, the roots are consumed and the tree may be girdled just under the soil surface (Figure 3). In beds of young seedlings, the plants may appear to be pulled deeper into the soil as the grubs feed. From time to time, the grubs emerge to the surface to deposit feces outside their burrow. In this process, they leave behind round holes in the surface that resemble puncture holes from a pencil.

Figure 3. Seedling with grub feeding damage (left) and healthy seedling (right). Courtesy of James Solomon, USDA Forest Service, Bugwood.org (#3066089)

As soil temperatures decrease in fall, grubs move lower in the soil. They burrow 8–10 inches (20.32–25.40 cm) below the soil surface to remain below the frost line overwinter. Japanese beetles only require 1 year to complete a life cycle, but members of the genera Phyllophaga and Polyphylla require up to 4 years.

Damage in seedling beds can occur as early as the first year after germination. However, in transplants, the most severe white grub damage generally occurs 2–3 years after field planting. This is when the grubs are mature and capable of severing the taproots. Plantings that were previously in pasture or grass are the most susceptible and, generally, the grubs were present before the trees were planted.

Monitoring and Management Strategies

Plantation Establishment

  • Corn fields are known reservoirs for Japanese beetles. Conifer fields planted adjacent to these could be more susceptible to grub damage.
  • Evaluate the present white grub population in early fall or spring before planting.
  • Dig a 10-foot-long (3.05 m) furrow. If one grub per 10 feet of furrow is found, consider treating before planting the following year.
  • Dig several square-foot holes and sift through soil to check for grubs. If more than one grub per hole is present, a preplanting treatment is recommended.

Preseason

  • No recommendations are available at this time.

Growing Season

  • Threshold levels are for grubs in soil prior to planting. See Plantation Establishment above.
  • At the end of the season, evaluate results and update records.

Control Options

Biological/Biorational

  • The bacterial powder Bacillus popillae causes a milky spore disease specific to Japanese beetle. Application to grassy areas in early summer or late fall allows grubs to ingest the bacteria, which will kill them before spring.
  • Pathogenic fungi, nematodes, and protozoans may help decrease grub populations.
  • The parasitic wasps Tiphia popilliavora (Rohwer) and Tiphia vernalis (Rohwer) are parasites of Japanese beetle and some other white grubs.
  • Birds and toads consume many beetles in addition to a small quantity of grubs.
  • Moles, shrews, and skunks feed heavily on grubs in the soil.

Mechanical

  • For low numbers of grubs, hand removal from tilled soil may be helpful.
  • For new seedling beds, till or disk several times in April–May or September to injure larvae in soil and expose them to parasite and predators.
  • Some states recommend maintenance of groundcover to offer alternate food sources to grubs in the plantation.

Chemical

  • Apply a registered insecticide with a stomach poison to grassy areas in the spring (March–mid-May) or fall (September–November) to control grubs.
  • Apply a registered insecticide as a preventative drench at the base of the plant, directed at the root ball/zone.
  • Root dip may be hard on trees, so in-field treatment is preferred.